The present invention relates to warning devices, and in particular to an a.c. powered fire detector having battery backup for providing power to the detector in the event of interruption of the a.c. power source.
Battery powered fire detectors for detecting the presence of products of combustion are self-sufficient for their power, and are advantageously used in situations either where a.c. power is not available, or where available a.c. power may reasonably be expected to be occasionally interrupted, particularly in the event of combustion. Such detectors may include means for supervising the energy level of the battery and for generating an indication, which is customarily an intermittent sounding of an audible alarm, upon the energy decreasing to a predetermined level to warn a user of the detector of the need for battery replacement. Should the detector be unattended for an extended period of time during which the low battery power indication is generated, it is possible for the battery to become completely depleted in generating the indication whereby the detector, when again attended, will neither provide the low battery power warning nor respond to products of combustion. Further, upon the occurrence of combustion, the detector generates an alarm, which is ordinarily a continuous sounding of the audible alarm, only for so long as the battery has sufficient power to operate the alarm, and thereafter becomes silent. As such detectors usually respond to the incipient stages of combustion, it is possible for the battery thereof to become completely exhausted in providing the alarm during the very early stages of combustion. Should the detector be unattended at that time, then a user thereof will receive no warning at a later time, even though the combustion may not yet have reached an advanced stage.
A.C. powered fire detectors depend for their operation upon an external source of a.c. voltage. As compared with battery powered detectors, there are no batteries to replace, and the detectors remain responsive to the presence of products of combustion to continue to generate an alarm for as long as the a.c. voltage is applied thereto. Unfortunately, unlike battery powered detectors, the a.c. powered detector is not self-sufficient for its power, and upon interruption of the a.c. voltage the detector becomes inoperative. Such interruption may result from normally occurring power failures, or as a consequence of the combustion itself, particularly where an electrical fire is involved.
Thus, neither battery powered fire detectors nor a.c. powered detectors may reliably be assumed to be continuous in their operation. It would, therefore, be extremely desirable to combine into a single detector the specific advantages offered by each of the battery and the a.c. powered detectors -- continuous operation for so long as a.c. line power is available, yet self-sufficiency in the event line power is interrupted.